For an operational principle of an optical device such as a scanner, light distances D and F respectively from a target object to a lens set and from the lens set to a photoelectric device of the optical device are both required to be constant. In view of a general design for reducing the size of an optical device, as shown in FIG. 1, a plurality of reflecting mirrors 11 are arranged between the target object 12 and the lens set 13. Light is transmitted from the target object 12 to the lens set 13 to be focused through the multiple reflections of the reflecting mirrors 11 with a light distance D equal to the distance of a+b+c+e, and then further transmitted to a photoelectric device 14 to be processed with another light distance F. By multiply reflecting light between the target object 12 and the lens set 13, the light distance D can be kept unchanged, and the distance between the target object 12 and the lens set 13 can be compacted from the distance D to a distance G much smaller than D. Accordingly, one of the dimensions of the optical device is reduced by at least a distance of (D+F)-(G+F).
In order to further reduce the size of an optical device, another kind of arrangement of reflecting mirrors is provided in another conventional optical device, as shown in FIG. 2. In this optical device, reflecting mirrors 211 and 212 allowing multiple reflections are used. Light is transmitted from the target object 22 to the lens set 23 to be focused through the multiple reflecting mirrors 211, 212 and 213 still with the constant light distance D which is equal to the distance of h+i+j+k+l+m in this case, and then further transmitted to a photoelectric device 24 to be processed with the constant light distance F. By this way, the light path is further compacted because of more reflections among the reflecting mirrors. The distance between the target object 12 and the lens set 13 is compacted from the distance G further to a distance N, and one of the dimensions of the optical device is accordingly further reduced by at least the distance of (G+F)-(N+F).
Although the size of an optical device can be reduced by the compact of the light path due to the arrangement of multiple reflecting mirrors, as shown in FIGS. 1 and 2, a part of the size contributed by the light path from a lens set to a photoelectric device cannot be reduced because the two light paths independently and separately exist in the optical device.